1. Field of the Invention
The invention relates to the testing and manufacturing of thin film materials. More specifically, the invention relates to thermal diagnostics for finding defects and/or measuring or identifying features of integrated circuits.
2. Background of the Invention
A variety of scanning thermal probes have been developed for mapping spatial variations in surface temperatures or the thermal properties of samples. The transducing elements for such devices have included thermocouples, Schottky diodes, bolometer-type resistance change devices, and bimorphs. A bolometer-type sensing element, which maps temperature by fractional changes in electrical resistance, has certain advantages for microcalorimetry applications. In particular, the resistor in the probe can be used to supply heat if sufficient current is passed through it. Because the tip temperature is ultimately influenced by the heat flow between the tip and the sample, variations in thermal conductance across the sample can be mapped by such a probe. If the heat is supplied by a periodic signal, local variations in thermal capacity can also be measured. In essence, because the probe tip serves as a point source of heat as well as a temperature sensor, such devices can be used as a spatially localized microcalorimeter. See A. Hammiche, et al., J. Vac. Sci. Technol. B, Vol. 14, 1996, pp. 1486, et seq.; L. E. Ocola, et al., Apl. Phys. Lett., Vol. 68, 1996, pp. 717, et seq.; D. Fryer, et al., Proc. SPIE, Vol. 333, 1998, pp. 1031, et seq.
While microcalorimetry techniques that use thermal probes for characterization of materials have been reported, they have failed to anticipate recent developments in both probe technology and rapidly evolving application areas. For example, past efforts in using thermally sensitive probes for microcalorimetry were focused on single point measurements that were made with either wire-based probes that were fabricated by non-lithographic means, or on lithographically micromachined single probes that were not appropriate for high-throughput measurements. In recent years, efforts at developing thermal probes using polyimide as the structural material have led to highly compliant devices that can be operated in contact with the sample surface without force feedback, even for relatively soft samples. In addition, the use of lithographic fabrication methods has permitted the integration of more sophisticated functionality than afforded by a single thermal sensor, and the development of more complex structures than a single cantilever or loop. In general, the thermal, mechanical, and electrical properties of the scanning microstructures can all be optimized for the particular application at hand. This evolution in microstructure technology has also opened up the possibility of using new sensing methods. For example, it is possible to stimulate a sample using one part or element of a probe-like microstructure, while the sensing is performed by a different part in the same structure or potentially even a separate microstructure that is coupled to the first in a pre-determined manner. As a further example, the nature of the electrical and thermal waveforms that may be used with these structures is more diverse than the time-invariant (fixed value) and the oscillatory waveforms that have been anticipated in the past. Thus, impulse-type waveforms that are not periodic may be used to determine the properties of a pixel or the simultaneous characterization of multiple disparate pixels. More complex waveforms are also anticipated. In addition, the potential application arenas for these systems have also changed. Several emerging needs for scanning thermal diagnostics in the semiconductor industry have not been anticipated or addressed in the past. These applications, which typically require high speed mapping of relatively large areas, and may sometimes require only qualitative comparisons, potentially include: the detection of spatially distributed defects in thin films such as sub-surface voids in interconnect metal or defects in the bond interface between two structures or materials; the characterization of process steps such as mapping the distribution of implanted dopant concentration or thickness variation in deposited films; and the correlation and comparison of simultaneously acquired high resolution images of temperature in an operating circuit or the observation of their temporal changes to predict or diagnose reliability. These various shortcomings in existing and previously anticipated know-how are addressed by the systems and methods that are described in this document.
The measurement of sub-surface areas with high speed, high spatial resolution and in a non-destructive manner is a significant challenge in semiconductor process monitoring. Voids in semiconductor surfaces arise to non-idealities in the deposition process. For example, voids may occur in thin film copper interconnects due to non-uniformities in deposition rates related to variations in topography or the proximity and size of features. It is not always possible to detect these defects before the completion of the manufacturing process, and they can potentially make their way into the final device or system, leading to long term reliability problems that are very expensive to detect and correct. Present options for detecting these defects include systems using: laser-induced surface acoustic waves, which have a resolution that exceeds 10 microns and so can only determine large clusters of defects; acoustic microscopy, which offers better resolution but can require the sample to be immersed in a liquid; point-by-point electrical testing, which lacks the necessary throughput to be practical in a production setting; and scanning electron microscopy, which requires the sample to be sectioned, and is accordingly both slow and destructive. These and other inadequacies of the prior art are addressed by this invention.
Dopant mapping in semiconductors is commonly performed by measuring the change in reflectance when the semiconductor is heated with a laser. For a given laser power output, the final temperature of the semiconductor depends upon the thermal conductivity, which corresponds to the dopant concentration. See A. Rosencwaig, Thermal Wave Characterization and Inspection of Semiconductor Materials and Devices, Ch. 5 in Photoacoustic and Thermal Wave Phenomena in Semiconductors, ed. by A. Mandelis (Elsevier Science Publishing, New York, 1987). Systems and methods based on scanning thermal microstructures potentially provide high resolution non-destructive alternatives to existing approaches and may even be used as complementary methods.
The thickness of a thin film on a substrate may be measured using photoacoustic methods. In this procedure, the sample is enclosed within a sealed container and illuminated with a laser through a variable speed optical chopper. The sample is heated by the chopped laser signal, causing the surrounding air to heat. The resulting increase in air pressure is measured. By varying the chopper frequency, the thickness of the thin film may be determined. See D. Almond, Photothermal science and techniques (Chapman and Hall, London) 1996. For this application as well, scanning thermal microstructures potentially provide high resolution non-destructive alternatives to existing approaches.